Aqueous bath for coloring stainless steel



Patented July 19, 1949 r 2,47 ,700 AQUEOUS BATH FOR ooLomNG STAINLESS STEEL Daniel .J. Clini, Springfield, Mass, assignor to Heatbath Corporation, Indian iii-chard, Mesa, a corporation of Massachusetts No Drawing. Application March 4., 1946. Serial No. 651.983

1 Claim. (01. 148 6-24) Th s inv nt n r ates to a bath compo o for tr ating that type of steel known as stain less, viz. containing at least 7% of chromium therein, for the purpose of imparting a black color to the surface of said stainless steel. The present application is a continuation-in-part of my application filed August 3, 1945, Serial No. 608,834, now abandoned.

The usual aqueous oxidizing baths which have been heretofore employed for blackening the surface of ordinary iron and steel are ineffective in black coloring the surface of stainless steel which is to be expected inasmuch as the stainless steels are designed for the purpose of resisting such oxidizing discoloration. An object therefore of my invention is to provide a bath composition which will give such reaction products as will produce a firmly adhering black coating on the surface of stainless steel. A further object of my invention is to produce such a stainless steel blackening bath as will have no deleterious corroding effect on other steel or iron parts which may accompany the stainless steel into the bath but will in fact serve to blacken ordinary iron or carbon steel parts along with the stainless steel.

In the preparation of the stainless steel surface for its treatment by my bath, the said surface should be thoroughly cleaned by any suitable commercial cleaner. This cleaning operation should remove all oil and grease from the steel surface. Thereafter the stainless steel surface is preferably subjected to a hot acid dip for slightly etching the same and rendering the steel surface receptive for the colored coating which will anchor itself thereto in the subsequent bath treatment. A preferred form of such acid dip is a hot aqueous solution of 10% sulphuric acid at a temperature of approximately 200 F.

This preliminary etching step is not essential to a my process but is found to accelerate the coloring action which later takes place in the bath. Other cleaning methods such as sand tumbling or sand blasting may also be employed to provide an effective surface activation of stainless steel in preparation for the operation of my 0010!- ing process.

In one form of my bath, the constituents are an alkali metal hydroxide, together with alkali metal salts, of a permanganate, of a sulphide and of a sulphate, all in aqueous solution. The exact proportions of the above compounds are not critical although it is important that the alkalinity of the bath shall be preserved and the hydroxide shall be in large proportion as compared with the other compounds. The per manganate is reduced more quickly to manganese dioxide in such an alkaline bath than in one which is acidic or neutral, and it is believed that manganesed-ioxide is one of the reaction prod ucts producing-the black coating on the stainless steel. Another possible reaction product from the above composition which assists in forming h -black coating, in mangancus sulphide. It is believed that the sulphate radical in the bath acts to give an etching effect on the stainless steel surface whereby the black coating compound cr compounds will take hold more readily and firmly.

Suitable proportions by weight for the aforesaid composition of my bath are as follows:

Parts Sodium (or potassium) hydroxide Potassium (or sodium) permanganate 2 Potassium (or sodium) sulphide 3 Sodium (or potassium) sulphate 1 and the balance water sufiicient to give a boiling point of approximately 250 F. i 20 F.

In another form of my bath I may obtain the sulphate radical therein by a substitution of manganese sulphate for the potassium permananate in the first named bath. In this second form of bath the composition by weight may be as follows: parts sodium (or potassium) hydroxide, 1 part manganese sulphate, and 1 part potassium (or sodium) sulphide with the balance water sufflcient to give a boiling point of approximately 250 F. 1: 20 F.

One of the features of my invention is that the bath at the relatively low temperature of 250 F. as compared with oxidizing baths, will be effective in black coloring stainless steel. In use, my bath is preferably kept boiling moderately and the control of the temperature is obtained by the salt concentration of the solution. If the bath boils below 250 F. water is allowed to evaporate or salts are added. If it boils above 250 F. water is added. The aforesaid temperature for the use of my bath is not critical and it will be found effective in approximate range between 230 F. and 270 F.

In the use of my bath, the stainless steel surface after thorough cleaning as above described, and after a brief preliminary etching in the 10% sulphuric acid dip (30 seconds to 1 minute), is then immersed in the bath of the composition described for 10 to 15 minutes. After removal therefrom, the surface is thoroughly rinsed, oiled, and dried in the usual manner. My composition when used in the manner described will serve to impart a black coating to various metals or alloys including all types of stainless steels from that with a low percentage of chromium content up to and including the austenitic types.

I have found that a water solution of a hydroxide, a sulphide, and a manganese compound is essential to produce a black coating of stain- 7 less steels. In one form of my bath the constituents are an alkali metal hydroxide, together with an alkali metal sulphide, an alkali metal permanganate, and an alkali metal sulphate.

I claim: a

A bath for imparting a black coating to stainless steel made up by adding together in aqueous solution a composition of a permanganate, a hydroxide, a sulphite, and a sulphate, each of said compounds having a metallic constituent chosen from the alkali metal group, said ingredients of the bath being added together in the approximate proportions by weight of 7 Parts Hydroxide 80 Permanganate 2 sulphide 3 Sulphate 1 and suificient water to give the bath a boiling point of about 250 F; v

, DANIEL J.

REFERENCES CITED The following references are of record in the file of this patent:

Chemical Abstracts, vol. 16 (1922), page 2458. Treadwell-Hall, Analytical Chemistry, vol. I,

Qualitative, seventh edition (1930), John Wiley 8: Sons, Inc., New York, pages 172 and 176.

Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 10, page 141,

Longmans Green & Co., New York, 1930. 

